Should I use #include in headers?

Question

Is it necessary to #include some file, if inside a header (*.h), types defined in this file are used?

For instance, if I use GLib and wish to use the

Answer 1

I use the following construct to be sure, that the needed include file is included before this include. I include all header files in source files only.

#ifndef INCLUDE_FILE_H
 #error "#include INCLUDE.h" must appear in source files before "#include THISFILE.h"
#endif